The proliferative response of -mammalian somatic cells to specific growth factors is mediated by signals transmitted from plasma membrane receptors to the nucleus. The long lag period between receptor stimulation and the proliferative cell response (usually hours) may reflect the complexity of intervening molecular events. The hypothesis to be tested is that the molecular basis for this signalling involves the sequential generation of a series of identifiable intermediate molecules during G1 of the cell cycle, that ultimately commit the nucleus to initiate DNA replication. To study the proliferative response of hematopoietic cells to interleukin-3 and serum growth factors at the molecular level, a cellfree system for growth factor-dependent initiation of DNA replication was developed. This model recapitulates the molecular events required for progression from early G1 to S phase of the cell cycle and is suitable for biochemical or genetic studies. The aims of the proposal are: (1) to further define the sequential requirements for growth factors, membrane and cytosolic subcellular fractions for initiation of DNA replication; (2) to produce a panel of inhibitory monoclonal antibodies to cytosolic proteins essential for growth factor-dependent initiation of DNA replication; (3) to identify and characterize intermediate signalling proteins downstream of IL-3 or serum growth factor receptors that are generated during G, of the cell cycle and essential for subsequent initiation of DNA replication. These aims will be accomplished by identifying cytosolic substrates of IL-3 receptor tyrosine kinase activity, cytosolic protein kinase C substrates, or other cytosolic proteins downstream of these molecules that bind to the nuclear DNA template and are essential signals for initiation of DNA replication. The long term goals of this study are to purify the most relevant or novel proteins involved in this signalling process, to clone and sequence their corresponding cDNA's, and define their function in the signalling process.